1. Fields of the Invention
The present invention relates to a control circuit and a method, especially to a control circuit and a method for maintaining light transmittance of an electrochromic device.
2. Descriptions of Related Art
By applying an external voltage, materials have the possibility to change their color reversibly. The materials are called electrochromic materials. Electrochromic materials have the property of color changing due to electrochemical redox reactions. They are transformed in a different charged state to obtain a different color. Thus the optical properties (color or transmittance) of the electrochromic device can be adjusted by the voltage applied. After being driven by a lower voltage, the electrochromic device is switched to a colored state. When the external voltage is off, ions are diffused slowly away from the active electrochromic layer. Thus the electrochromic device is switched to a bleached state. By applying positive and negative potential alternately, electrochemical reactions occur in the electrochromic device. The color is changed between the colored state and the bleached state. The efficiency of coloration and bleaching of the electrochromic device depends on the ion movement in the electrochromic layer.
In response to the external voltage applied, the electrochromic materials change light transmittance properties. The electrochromic materials also have memory effect and reversibility of electrochromism. Thus the incidence of light or other electromagnetic radiation with different wavelengths can be controlled and adjusted. Therefore the electrochromic materials can block some wavelengths of light, control the amount of light transmission and save energy used. The electrochromic materials have attracted considerable attention recently.
However, the longer the voltage is applied, the more ions are transferred and the darker the electrochromic material is. The light transmittance is reduced. Thus the electrochromic materials are not suitable for special glass on vehicles that requires good transmittance. There is room for improvement.